A former Chechen police officer who admitted to taking part in the murder of Russian opposition activist Boris Nemtsov "likely confessed under torture", a member of the Kremlin's rights council told AFP Wednesday.

"There are reasons that lead us to believe Zaur Dadayev confessed under torture," said Andrei Babushkin, adding he had seen "numerous wounds" on the body of the prime suspect in the killing during a visit to his Moscow prison cell on Tuesday.

Zaur Dadayev, a former deputy commander in a special Chechen police unit, was charged Sunday with the brazen murder of Kremlin critic Nemstov alongside Anzor Gubashev who worked for a private security company in Moscow. They, along with three other suspects, were remanded into custody.

A court in Moscow heard the men were being probed under a section of the Russian criminal code relating to murders carried out for financial gain, in a sign investigators believe Nemtsov's murder was a hit.

But Babushkin said Dadayev claimed he had made the confession under duress after being arrested in the North Caucasus region of Ingushetia last week, alleging that he spent two days manacled and with a sack over his head.

"They shouted at me all the time, 'You killed Nemtsov, didn't you?'. I said, no," Babushkin reported Dadayev as saying.

The suspect said he had eventually admitted to the killing to secure the release of an ex-colleague, Ruslan Yusupov, who was detained alongside him.

"They said that if I confessed they would let him go. I agreed. I thought I would save him and they would bring me to Moscow alive," Dadayev said, according to Babushkin.

The slaying of Nemtsov, a former deputy prime minister who became an outspoken foe of President Vladimir Putin, just yards from the Kremlin was the highest-profile killing of an opposition leader during Putin's fifteen years in charge.

The 55-year-old Nemtsov was shot four times in the back on February 27 as he walked along a bridge in central Moscow with his girlfriend.

The killing sent shivers through an opposition that accuses Putin of steadily suppressing dissent, and accused the Kremlin of being behind the murder of one of its last outspoken critics.

Investigators have said they were also probing the possibility he was assassinated for criticising Russia's role in the Ukraine conflict or as part of a plot to destabilise the country.